How would I calculate this?

Question

Hi,

How would I calculate this:

How many NaCl formula units does 20mg of salt crystal contain?

(Na - 22.9 amu Cl - 35.45 amu)

Thank you!!

Answer 1

first.  the formula weight of NaCl...

NaCl has 1 Na and 1 Cl ---> 1 x 22.99 + 1 x 35.45 = 58.44 amu / unit

next

1 gram = 1.66 x 10^24 amu

1 gram = 1000 mg

so here we go...

20 mg x (1 g / 1000 mg) x (1.66 x 10^24 amu / g) x (1 unit / 58.44 amu)

= 5.68 x 10^20 formula units

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let me know if you need me to explain this calculation...

Answer 2

i think omega was close.  since you have .020 g NaCl and the formula weight is 58.35g/mol, the answer should be

0.020g / (58.35g/mol ) = 3.4x10^-4 mol NaCl

edit: two points.

A) sorry, sophiee - mw is right - you can also express the answer in actual units by multiplying by NA,Avogadro's constant:

(3.4x10^-4 mol) * (6.022x10^23 units/mol) = 2.1x10^20 formula units

B) mw, i think you had a typo, since 1amu = 1/NA=1.66 x 10^-24g.  if you go through and correct the algebra, our answers match up.

Answer 3

not quite sure but :

Na+Cl = 58.35



20mg=20x10^-3 g

x=20x10^-3x58.35 / 1 = 1.167